Two phonon vibration spectra of amorphous GeS and GeSe systems

Journal of Non-Crystalline Solids 77 & 78 (1985) 1121-1124 North-Holland, Amsterdam

l 121

TWO PHONONVIBRATION SPECTRA OF AMORPHOUSGe-S AND Ge-Se SYSTEMS Seinosuke ONARI, Kiyote MATSUISHI and Toshihiro ARAI I n s t i t u t e of Applied Physics, University of Tsukuba, Sakura, Ibaraki 305, Japan Two phonon Raman spectra of GexSI x (x= 0.25, 0.33, 0.37 ) and Ge Se~ x (x=O, 0 . I , 0.2, 0.3 and 0.33 ) a~e reported, and the local struct~re~of these glasses are discussed by the comparison of the f i r s t and the second order Raman spectra. The absence of the combination tones with the SR ring vibration suggests that S8 ring molecule exists isolately. CompaMion Al band of GeS2 is shown to combine with S-S stretching vibrations. I. INTRODUCTION The physical properties of Ge-chalcogenide glasses have attracted much interest for physical and technical reasons. Many researchers have been interested in the mediumrange orderI of the network of the GeS2 and GeSe2

,

glasses especially concerning the existence of Se-Se bonds in the r a f t model. The f i r s t order infrared and Ramanspectra 2'3'4 and neutron scattering etc. have been studied in order to obtain the information about the local and intermediate range orderings. Two phonon infrared and Raman spectra of Aschalcogenides have been reported5'6'7. In this report the two phonon Ramanspectra of Ge-S and Ge-Se systems are studied. From the selective properties of the two phonon Raman lines, local structures and some characteristic properties concerning the medium range ordering are discussed. 2. EXPERIMENTAL The samples used were prepared by the melt quenching of a mixture of the elements Ge, S, and Se of high purity (6N) for each compositions. In order to obtain the homogeneoussamples, a rocking furnace was used. Raman spectra were measured with a Jobin Yvon l m double monochrQmator U-lO00 with holographic gratings in a standard back scattering geometry. A Gran-Taylor prism polarizer was used for the selection of the l i g h t polarization. A photon counting system was employedusing an RCA C31034 photomultiplier as a detector, and a Kr+ ion laser line of 6471A was used as the l i g h t source. 3. RESULTS AND DISCUSSIONS The two phonon Raman scattering spectra of GexSl x systems have dominant bands in the energy range from 600 cm-l to lO00 cm-T, and their spectra change with the change of the composition x.

As the wave numbers of the Raman spectra

0022~093/85/$03.30 © Elsevier Science Publishe~ B.V. (North-Holland Phy~csPublishm~ DNision)

S. Onari et al. / Two phonon vibration spectra

1122

in this region is almost equal to the double value of the f i r s t order Raman scattering in the region of 300 cm-l to 500 cm- l , these bands may surely be the two phonon Raman bands. Whenthese two spectra are drawn in the same figure with the double value of the energy scale for the f i r s t order spectra, the spectral shape is, however, not always similar to each other. The second order Raman intensity for the system without translational 5 symmetry may be represented for Stokes scattering in the semiclassical form as I(~) = f d m '(.n- (- ~ ~ '_) + ( l ) n(~-~' ~-m !+I )p(~, )p(~_~, )C(~,~, )

(I)

where n(m) is the Bose-Einstein statistical factor, C(m,m') is an effective coupling parameter which reflects the modulation of the electric susceptibility by the phonon amplitudes, p(m) is the one phonon density of states, and co is the Raman shift. -'T

I

'

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~:2nd ..... :1st :Convol.

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X:0.25 600 800 1000 1200 2nd 300 400 500 600 Ist WAVE NUMBER ( c niI ) FIGURE 1(a) One and two phonon Raman spectra of GexSt_x (a) and GexSel_x (b) and the conbolution spectra by the equation (1).

X:O.O ,

I 400 600 2nd 200 300 1st WAVE NUMBER (crrT 1) FIGURE 1 (b)

S. Onari et al. I Two phonon vibration spectra

1123

The convolution spectra calculated from the equation (I) are shown in Fig. 1 (a) and (b), and there is somedeviation from the observed two phonon spectra. The f i r s t order Raman spectrum of Ge25S75 is composedof Al band of GeS4 tetrahedron, companion Al band, F2 band of GeS4 tetrahedron, vibration of S8 ring molecules, and the stretching vibration of S-S bonds which have the intimate relation with the presence of GeS4 tetrahedron, and these band are labelled in Fig. 2 by O, P, Q, R, and W , respectively. As is shown in Fig. 2, the combination bands of O+P, P+Q, O+W, P+W and Q+W are observed. As the group theory predicts that the overtone and combination tones of Al , F2, and E modes of GeS4 molecule are allowed in the Raman spectra, the relaxation of the one phonon Raman selection rules is expected in the amorphous systems. I f the overtone mechanism is predominant, the two phonon Ramanspectra may therefore reflect the one phonon density of states. In order to see the contribution of the combination tones, weighted overtone bands 0', P', Q', R' and W' are subtracted from the measured two phonon Raman bands. The resulting Raman bands are explained well as the combination tones of O+P, O+Q, P+Q, O+W, P+W and Q+W as shown in Fig. 2. i

I

i

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fi

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FIGURE 2 One and two phonon Raman spectra of Ge25S75 and the decomposition and combination spectra.

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FIGURE 3 One and two phonon Raman sepctra of GeSe2 and the decomposition spectra.

1124

S. Onari et al. / Two phonon vibration spectra

The fact that the combination tone with R band is not observed in the two phonon spectra suggests that the S8 ring exists isolated from the GeS4 tetrahedron. Wmode, however, combineswith the GeS4 tetrahedron vibration modeof O, P, and Q band. The existence of the combination tome of P+W band suggests that the vibrating unit of the companion Al band may be connected with the S-S bonds. As shown in Fig. 3 for GeSe2, Alg band of Ge2Se6 ethane molecule, Al band and A1 companion band of GeSe4 tetrahedron appear in the range of 170-230 cm- l , and they are here labeled U, S, and T, respectively.

In the two phonon Raman

spectra of the energy range 340-460 cm- l , overtone of S, T, and U bands and the combination of S with T are observed. However, corresponding to the following situation that, the Ge2Se6 ethane molecules do not constitute the GeSe4 tetrahedron, the combination tone of U with S or T are not observed. A rather strong broad two phonon Raman band with some structures are observed in the energy range 450 cm-l-650 cm-l for amorphous GeSep, however, the one phonon Raman scattering intensities in the energy range 225~cm-I-325 cm-l are rather weak due probably to the selection effect.

Somepeaks and shoulders

of the band in the region of 450-650 cm-l correspond to the overtones of the peaks in the f i r s t

order Raman spectra and that i f peaks in ~ 2 ( ~ ) and ~Im(-I/E)

spectra derived from the r e f l e c t i o n spectra.

These facts suggest that the

matrix elements e f f e c t on the two phonon Raman coupling constant C is weak and the selective e f f e c t on the two phonon spectra relaxes f a i r l y with the one phonon spectra.

in comparison

The observed two phonon spectra can be explained

by the superposition of the overtone bands and some selective combination bands rather than the self-convolution of the one phonon sepctra. REFERENCES l) J.C.Phillips, J. Non-Cryst. Solids 43 (1981) 37 2) G.Lucovsky et al. Phys. Rev. BIO (1974) 5143 3) N.Kumagai et al. J. Phys. Soc. Japan 42 (1977) 1262 4) R.J.Nemanich, et al. Sol. State Commun. 21 (1977) 273 5) J.S.Lannin and P.J.Carroll, Philos. Mag. B45 (1982) 155 6) S.Onari, H.Kataura, and T.Arai, Jpn. J. Appl. Phys. 21 (1982) 1566 7) S.Onari, K.Matsuishi, and T.Arai, J. Non-Cryst. Solid in press